U.S. application Ser. No. 2002/0186214A1, by Siwinski, published Dec. 12, 2002, shows a method for saving power in an organic light emitting diode (OLED) display having pixels comprised of red, green, blue and white light emitting elements. The white light emitting elements are more efficient than the other colored light emitting elements and are employed to reduce the power requirements of the display.
OLED devices age as current passes through the emissive materials of the display. Specifically, the emissive materials age in direct proportion to the current density passing through the materials. Hence, the solution proposed by Siwinski will have the effect of either reducing the size of the emissive elements (if four elements occupy the same area as three elements), or reducing the resolution of the device (if four elements take more area than three elements). Therefore, the design of Siwinski will result in either reduced lifetime or reduced resolution compared to a prior art three element design.
One approach to dealing with the aging problem, while maintaining the resolution of the display, is to stack the OLED light emitting elements on top of each other thereby allowing the areas of the light emitting elements to be larger to improve lifetime, and/or allowing more pixels to be provided for a given area, thereby improving resolution. This approach is described in U.S. Pat. No. 5,703,436 issued Dec. 30, 1997 to Forrest et al. and U.S. Pat. No. 6,274,980 issued Aug. 14, 2001 to Burrows et al. Stacked OLEDs utilize a stack of light emitting elements located one above another over a substrate. Each light emitting element is individually controlled using conventional controllers. Power is supplied to the light emitting elements from the controller through transparent electrodes which may be shared between light emitting elements adjacent to each other in the stack. However, such stacked structures do not improve the efficiency of the pixels in the display.
It is also known that different OLED materials for emitting different colors of light age at different rates as they are used. It is has been proposed to provide an OLED display having pixels with differently sized red, green and blue light emitting elements, wherein the relative sizes of the elements in a pixel are selected according to their relative aging characteristics to extend the service life of the display. See U.S. Pat. No. 6,366,025 B1, issued Apr. 2, 2002 to Yamada.
It has been known for many years that the human eye is most sensitive to green light and less sensitive to red and blue light. More specifically, the spatial resolution of the human visual system is driven primarily by the luminance rather than the chrominance of a signal. Since green light provides the preponderance of luminance information in typical viewing environments, the spatial resolution of the visual system during normal daylight viewing conditions is highest for green light, lower for red light, and even lower for blue light when viewing images generated by a typical color balanced image capture and display system. This fact has been used in a variety of ways to optimize the frequency response of imaging systems. For example, as described in U.S. application Ser. No. 2002/0024618 by Imai, published Feb. 28, 2002, in a pixel having a square array of red, green, blue and white light emitting elements, the colors green and white having large luminance components are positioned diagonally opposite in the array. However, the Imai design does not provide increased power efficiency for an emissive full color display.
There is a need, therefore, for an improved full color flat panel OLED display having improved lifetime and power efficiency and a simpler construction.